Among the impact mills which have been developed for the comminution of mineral matter, there are those which have been referred to as "pin mills", e.g. of the type described on pages 8-37 and 8-38 of Chemical Engineers' Handbook, McGraw-Hill Book Company, 5th edition, 1973, New York. These mills, as a class, may be described as having a mill housing and two counterrotating rotor discs which are formed with the milling tools generally in an angularly equispaced relationship and in concentric circles so that the tools, generally pins, of the two discs interdigitate. Within each circle of tools the pins are equispaced and the number of pins per circle can vary from an inner part of the disc to an outer part thereof.
The material to be milled can be fed to the milling space within the circles of pins and the milled material is progressively cast outwardly and is discharged at the periphery of the space.
As the milled material is progressively cast outwardly it has increasing fineness and thus the inner pins bring about a coarse milling while the intermediate pins effect an intermediate fineness of milling and the peripheral pins give rise to the ultimate fineness of the product.
The mineral material which can be comminuted by such impact mills includes coal, oil shale, metallurgical ores and the like and, for stablization, the pins of a given circular array can have their ends remote from the respective disc bridged by respective stabilizing rings.
It has been found to be advantageous to provide the milling members or pins with partitions which extend radially with respect to the pins and define compartments associated with the pins.
Such mills are described in, for example, German patent document--open application DE-OS 16 07 582, in which the structure is similar to that of a conventional pin mill and in German patent documents--open applications DE-OS 29 26 042 and DE-OS 29 33 592 which describe systems for reducing the wear by the use of such radial partitions to define compartments.
Because of the formation of these compartments, it appears that the particles of the milled product as they are encountered by each milling tool do not abrasively impact upon the metal of the tool itself, but rather impact upon other particles, generally previously comminuted, in a cushion of the milled product entrained with each milling tool.
Consequently, the impact mill operates primarily by entrainment of a mass of the particles and the impact of this mass on other particles rather than primarily via inpact of the metal pins against the particles so as to minimize wear of the pins or more generally the milling tools. The impact milling operation is an entropy generating process with only part of the energy input resulting in the mechanical size reduction of the product. The balance of the energy is transformed into heat and it is always desirable in such system to maximize the fraction of the energy which is utilized in comminution as opposed to unproductive displacement, turbulence or the like.
Generally, the partitions are oriented at an angle of 45.degree. to the direction of rotation of the discs, an angle which presumably has been found to be advantageous for the impact comminution effect. In practice, results obtained with such systems are poor and we have discovered that these results are, in large measure, a result of the poor aerodynamics of the system which causes this system to be high in energy loss and hence are energy inefficient.